Method for anesthetizing aquatic animals

ABSTRACT

An improved method for anesthetizing aquatic animals which comprises anesthetizing the animals in water with nitrous oxide and carbon dioxide.

FIELD OF THE INVENTION

The present invention relates to a novel method for anesthetizingaquatic animals. More particularly, the method of the present inventionutilizes a combination of nitrous oxide and carbon dioxide and isapplicable to transport of live fish and other valuable aquatic animals,selection of cultivated fry, adjustment of spawning seasons of fish andthe like.

BACKGROUND OF THE INVENTION

Hitherto, as a method for transporting live fish and the like, there hasbeen mainly employed such a method that they are put in a water tankhaving a large volume and are transported with the entire tank. However,this method requires a high transport cost and, in order to reduce thecost, various methods have been tried. For example, lots of fish aretransported in sea water or fresh water with cooling by addition of iceor the like to lower movements of fish and to make the range of theirmovements small. Or, several bamboo baskets each of which contains onefish are piled up and the entire baskets are placed in sea water orfresh water to limit movements of fish.

Further, in Japanese Patent Kokai No. 51-103000, there is described amethod for transporting fish wherein fish are anesthetized with ananesthetic and cooled in a water tank, and then transferred into anotherwater tank containing no anesthetic. In "Possibility of ApplyingAnesthesia by Carbon Dioxide in the Transportation of Live Fish", Bull.Jap. Soc. Sci. Fish, 49 (5), 725-731 (1983), there is described a methodusing NaHCO₃ and an acid as well as a method employing aeration with CO₂and O₂.

On the other hand, in the selection of fry, there has been known amethod wherein fry are anesthetized by bubbling CO₂ gas through seawater to lower pH. Among fry, there are malformed fry having no ordeformed bladders by nature, which frequently results in shapeabnormality after grown up. Accordingly, they should be thinned out atthe initial stage of cultivation. However, there is no differencebetween the ordinary state of movements of malformed fry and that ofnormal fry, and it is very difficult to differentiate such malformed fryfrom normal fry by appearance. Provided that, when fry are anesthetized,there is observed such a phenomenon that normal fry rise to the surfacewith facing their bladders upward, while malformed fry sink below thewater. Thereby, whether fry are normal or abnormal can be readilyjudged, and the selection of fry can be readily conducted. In this knownmethod, CO₂ is bubbled into sea water to lower pH to the critical value(usually 5.6) or lower to anesthetize fish forcibly.

In this conventional aeration with CO₂, it is known that, when pH dropsto a certain critical value, fish show vigorous swimming motions justbefore they are put under anesthesia, although it is varied according tokinds of fish. This is known as a so-called "exaltation phase at initialstage under anesthesia". As described in the above Bull. Jap. Sci. Fish,49 (5), in general, when a drug is effected on fish, the followingstages are usually observed in order according to the degree ofintensity of its activity.

(1) Fish show vigorous swimming motions with so-called "surfacing" likebehavior.

(2) Fish response to stimulation becomes very weak but fish keep theirbalance. Respiration movements are deep and hard.

(3) Fish lose their balance and lie on their sides to almost repose.Respiration movements are deep and hard.

(4) Fish lie on their sides in repose. Respiration movements are lightand hard.

(5) Fish lie on their sides in repose. Respiration movements areirregular.

(6) Fish lie on their sides in repose. Respiration movements cease todie.

These have been also confirmed by the present inventors' experimentsand, when fish show vigorous swimming motions, sometimes, they jump upabout 30 cm or more above the water and often damage themselves byhitting against the edge or wall of a water tank. It is clear that thisdecreases in value of live fish, and therefore, this stage is a problemin anesthesia of fish. However, it is considered that this stage isunavoidable in a method of anesthetizing fish by aeration with CO₂.

When transporting live fish, it is necessary to maintain fish in thestate of the above (3) and, in addition, it is desirable to directlyintroduce fish to the above state (3) without passing through the abovestates (1) and (2). Accordingly, in transporting live fish, the abovevarious methods have been tried. However, they are still insufficientbecause, in the method using an anesthetic, it is required to keep awater temperature low or, in CO₂ aeration, the above damage of fish isunavoidable. Therefore, the above methods are not widely employed inpractice. Particularly, in the case of very valuable high grademarketable fish such as red sea bream or aquarium fish such as coloredcarp, CO₂ aeration can not be employed in practice because, as describedabove, fish are liable to be damaged by this method and damage in thesekinds of fish remarkably reduces their value. Therefore, it has beenrequested in this field to develop a method for anesthetizing fishwithout damage thereof.

Besides fish, the similar problems are present in transporting otheraquatic animals such as squids, cuttlefish, octopuses, prawns, lobsters,crabs and other valuable high grade aquatic animals because there is aremarkable difference in commercial values between transporting themwith keeping alive and transporting them without keeping alive such asfrozen products. Therefore, it is desirable to transport them withkeeping alive. Particularly, when squids or octopuses are transported byputting them in a water tank, there is such a problem that they hittheir heads against the wall of the tank to die, or show vigorousswimming motions and eject ink to contaminate water. This requirescleaning means such as filtration or adsorption means. Thereby, anadditional cost is required and, further, a yield is lowered.

OBJECTS OF THE INVENTION

The present inventors have studied intensively to solve the aboveproblems in a conventional anesthetizing method of fish. As the result,it has been found that, when aeration with nitrous oxide (N₂ O) iscombined with aeration with CO₂, fish are anesthetized without passingthrough an "exaltation phase at initial stage under anesthesia", i.e.,without damage due to vigorous swimming motions. The "combination ofaeration with N₂ O and aeration with CO₂ " includes simultaneousbubbling of N₂ O and CO₂ into water containing fish from separate gassources. Alternatively, a mixed gas of N₂ O and CO₂ can be bubbled intowater containing fish, or fish can be put in water into which N₂ O andCO₂ has been previously bubbled. Or, fish can be put in water into whichN₂ O has been previously bubbled and then CO₂ is bubbled into water. Inbrief, the "combination" means the use of both N₂ O and CO₂ in wateruntil fish are put under anesthesia.

Further, it has been also found that this combined aeration with nitrousoxide and CO₂ is effective for anesthetizing not only fish, i.e., aimalsbelonging to the class Pisces but also all other aquatic animalsincluding animals belonging to the classes Cephalopoda, Pelecypoda, andCrustacea.

That is, the main object of the present invention is provide a novelmethod for anesthetizing aquatic animals which can be readily practicedat a low cost with minimizing problems in a conventional method such asdamage of the animals.

This object as well as other objects and advantages of the presentinvention will become apparent to those skilled in the art from thefollowing description by reference to accompaying drawings.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating one embodiment of the methodof the present invention as described in Example 1 hereinafter.

FIG. 2 is a schematic diagram illustrating another embodiment of themethod of the present invention as described in Example 4 hereinafter.

FIG. 3 is a schematic diagram illustrating still another embodiment ofthe method of the present invention as described in Example 6hereinafter.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method foranesthetizing aquatic animals which comprises anesthetizing the animalsin water with nitrous oxide and carbon dioxide. More paricularly, themethod of the present invention characterized by anesthetizing aquaticanimals in water with carbon dioxide in the presence of nitrous oxide.

In comparison with a conventional method using an anesthetic or CO₂aeration, the method of the present invention can effectivelyanesthetize aquatic animals by a simple operation. Further, in themethod of the present invention, damage of animals due to vigorousswimming motions before they are put under anesthesia can be minimizedand animals are readily awakened from anesthesia by putting into freshsea water or water preferably enriched with oxygen. As the result, it ispossible to markedly reduce transport cost or cost for the selection offry.

DETAILED DESCRIPTION OF THE INVENTION

In the method of the present invention, aquatic animals are anesthetizedby bubbling a gas or gases containing N₂ O and CO₂ into water containingthe animals through one or more aeration pipes such as those having asintered glass tip or an air stone tip to absorb them into water.

Regarding the effects of N₂ O and CO₂ on aquatic animals, it isconsidered that N₂ O exerts sedative activity and CO₂ exerts anestheticactivity. It is impossible to anesthetize animals without vigorousswimming motions only by either of the above activities. When the bothactivities are combined, aquatic animals can be anesthetized for thefirst time without vigorous swimming motions as the result of theirsynergism. This has not been found heretofore in the prior art and isone of important advantages of the present invention. Although it iswell known that N₂ O is an anesthetic gas and is often used in anoperation of the human body, there is no case that N₂ O is applied to anaquatic animal. That is, according to the method of the presentinvention, N₂ O is firstly applied to aquatic animals in combinationwith CO₂.

The aquatic animals to which the method of the present invention isapplied is so-called primary aquatic animals which are cold bloodedanimals living in water and respiring dissolved oxygen. Particularly,the method of the present invention is preferably applied to veryvaluable high grade marketable animals from the economical point ofview. Examples of such animals include those belonging to the classPisces such as salmon, trout, char, ayu, carp, crucian carp, goldfish,roach, whitebait, eel, conger eel, sardine, flying fish, sea bass, seabream, parrot bass, snapper, mackerel, horse mackerel, tuna, bonito,yellowtail, rockfish, fluke, sole, flounder, blowfish, filefish, etc.;those belonging to the class Cephalopoda such as squid, cuttlefish,octopus, etc.; those belonging to the class Pelecypoda such as clam,scallop, ark shell, oyster, etc.; those belonging to the classGastropoda such as turban shell, abalone, etc.; and those belonging tothe class Crustacea such as lobster, prawn, shrimp, crab, squilla, etc.

By appropriately selecting conditions such as a flow rate of gas to bebubbled, a ratio of N₂ O to CO₂, an aeration time and the like accordingto kinds and size of animals to be applied, desired sedative activity ofN₂ O and anesthetic activity of CO₂ can be exerted to obtain a desiredeffect of the present invention. Although such conditions are variedaccording to size of a water tank, one standard for conditions in theanesthetizing method of the present invention is as follows. Whenanesthetizing sea bream (Chrysophrys major) having 1 kg of body weightin a 650 liter water tank, N₂ O and CO₂ are bubbled into sea water atthe flow rates of 10 liter/min. and 1 liter/min., respectively, for 20minutes or more to bring pH to 6.0 or lower. Thereby, sea bream isanesthetized calmly.

The method of the present invention can be carried out, for example, byputting aquatic animals such as fish in a water tank and bubbling N₂ Oand CO₂ into water from different nozzles simultaneously under the aboveconditions, or by preparing a mixed gas of N₂ O and CO₂ and bubbling themixed gas into water from one nozzle. Alternatively, it is possible toput aquatic animals in a water tank into which N₂ O and CO₂ have beenpreviously bubbled. Further, it is possible to firstly bubble N₂ O andthen CO₂ into water, or to put aquatic animals in a water tank intowhich N₂ O has been previously bubbled for a certain period of time andthen transfer the animals to another water tank into which CO₂ isbubbled. These are appropriately selected according to a particular useof the method of the present invention. That is, in the method of thepresent invention, it is necessary that sufficient sedative activity byN₂ O should be exerted on aquatic animals before exertion of anestheticactivity by CO₂ due to lowering of pH of water. In other words, theaeration ratio of N₂ O to CO₂ is of importance in the case that aerationis carried out simultaneously, or the mixing ratio of N₂ O and CO₂ is ofimportance in the case that a mixed gas is used.

Although the optimum ratio of N₂ O to CO₂ is varied according to kindsof aquatic animals, difference between individuals, water temperatureand the like, in general, the ratio of CO₂ :N₂ O is in the range of,preferably, 1:1 to 1:100, more preferably, 1:8 to 1:30. Even if N₂ O isincreased more than this range, no more sedative effect before exertionof anesthetic activity by CO₂ can be expected and therefore it isdisadvantage from the economical point of view. On the other hand, whenCO₂ is increased, pH is dropped to the prescribed point before exertionof sedative effect by N₂ O, which results in the "exaltation phase atinitial stage under anesthesia" of the animals to cause vigorousswimming motions. This is undesirable.

Particularly, in the case of anesthetization of sea bream, blowfish,flounder, lobster and the like, aeration is carried out at the CO₂ :N₂ Oratio of 1:8 to 1:10 until pH becomes 6.2 to 5.8. In the case ofanesthetization of yellowtail, filefish, squid and the like, aeration iscarried out at the CO₂ :N₂ O ratio of 1:10 to 1:30 until pH becomes 6.8to 6.3. Thereby, it is possible to exert desired sedative and anestheticactivities smoothly.

By the way, dissolved oxygen may be decreased by bubbling of these gasesinto water to cause an oxygen deficit state, which results in weakeningof the animals. Therefore, it is desired to also bubble oxygen intowater in such a manner that dissolved oxygen in water is kept in aconcentration of not less than 5 ppm, for example, by monitoring theconcentration with a dissolved oxygen (DO) meter.

The method of the present invention can be carried out at ambienttemperature and cooling of water is not required.

In order to awaken the animals from anesthesia, for example, fish underanesthesia are transferred to another water tank into which N₂ O and CO₂are not bubbled. Preferably, at this stage, oxygen is bubbled into waterto increase dissolved oxygen. Thereby, fish can be readily awakened fromanesthesia within a short period of time, for example, several minutesto several ten minutes.

The following Examples further illustrate the present invention indetail but are not to be construed to limit the scope thereof.

EXAMPLE 1

In this Example, anesthetization is carried out by using an apparatus asshown in FIG. 1.

In the apparatus of FIG. 1, N₂ O gas and CO₂ gas are bubbled into a seawater tank (1) containing aquatic animals from a N₂ O bomb (2) and CO₂bomb (3) each of which has a control valve and a flowmeter through anaeration pipe (4). O₂ gas is also bubbled from a O₂ bomb (5) through anaeration pipe (6). The tank (1) is equipped with a DO meter (7) and a pHmeter (8) to monitor pH and DO conditions.

About 600 liter of sea water was filled in the 650 liter tank (1), andtwo yellowtails (Seriola quinqueradiata, about 5 kg of body weight), twosea breams (Chrysophrys major, about 1 kg of body weight) and twoflounders (Paralichtys olivaceus, about 700 g of body weight) were putin the tank. The tank was aerated with 10 liter/min. of N₂ O and 1liter/min. of CO₂ as well as O₂ with monitoring DO meter (7) so as tokeep dissolved oxygen at a concentration of at least 7 to 8 ppm.Behavior of fish were observed according to change of pH. The initial pHof sea water was 7.8 to 7.9 and the initial water temperature was 20° C.The relation between aeration time, pH and DO is shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Time (min.)                                                                           0     5      6   8   10   15   19    22   25                          pH      7.8   7.0    6.5 6.4 6.3  6.1  6.0   6.0  6.0                         DO (ppm)                                                                              5.1   6.5    6.9 7.0 7.3  7.9  8.2   8.3  8.6                         ______________________________________                                    

From immediatley after commencement of aeration, among the fish,yellowtails began to show "surfacing". After 6 minutes, one ofyellowtails was put under anesthesia at pH about 6.5 and the other onewas also put under anesthesia at pH 6.3 after 10 minutes. They rose tothe surface with lying on their backs. Regarding sea breams, after 15minutes, they showed "surfacing" at pH about 6.1 and, after 19 minutes,two sea breams were put under anesthesia at pH about 6.0. They lay ontheir sides and rose to the surface. Regarding flounders, since they layon the bottom of the tank without significant motions from the beginningof aeration, it was difficult to confirm when they were put underanesthesia. However, it was considered that they were put underanesthesia at pH about 6.0 because, when they were pecked with a stickand turned over at this pH, they did not return to the originalpositions.

EXAMPLE 2

About 350 liter of sea water was filled in the 400 liter tank as shownin FIG. 1 and two filefishes (Stephanolepis cirrhifer, about 20 cm ofbody length), two squid (Dorytenthis kensaki, about 20 cm of bodylength) and two lobsters (Panulirus japonicus, about 20 cm of bodylength) were put in the tank. The tank was aerated with 8 liter/min. ofN₂ O as well as CO₂ with adjusting the flow rate in the range of 0.35 to1.5 liter/min. according to pH drop. O₂ was also continuously bubbledinto sea water with monitoring by DO meter (7) so as to keep dissolvedoxygen at a concentration of at least 7 to 8 ppm. The initial pH of seawater was 8.0 and the initial water temperature was 19.5° C. The changesof the flow rate, pH and DO are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Time (min.)                                                                             0     4      7    9    14   25   34  50  56                         N.sub.2 O (liter/min)                                                                   8.0   8.0    8.0  8.0  8.0  8.0  8.0 8.0 --                         CO.sub.2 (liter/min)                                                                    1.0   0.35   0.35 0.35 0.35 1.0  1.5 1.5 --                         pH        8.0   7.1    7.0  6.8  6.7  6.7  6.4 6.0  5.9                       DO (ppm)  9.7   10.9   10.6 10.3 8.6  14.1 8.6 7.5 12.0                       ______________________________________                                    

After 4 minutes from commencement of aeration, filefishes began to show"surfacing" at pH of about 7.1. The body color became reddish and, after9 minutes, they rose to the surface at pH about 6.8. After 14 minutes,they were completely put under anesthesia without vigorous swimmingmotions. Regarding squids, after 7 minutes, one of them rose to thesurface at pH 7.0 and put under anesthesia with closing eyes. The otherone was also put under anesthesia calmly after 14 minutes withoutvigorous swimming motion or ejection of ink. Regarding lobsters, after14 minutes, they began to show such behavior that they moved backwardwith enfolding their tails under abdomens, which is characteristic ofanesthetic effect, at about 6.4. After 56 minutes, it was confirmed thatthey were put under anesthesia calmly at pH of about 5.9.

EXAMPLE 3

The same tank as used in Example 1 was used and two sea breams (1 kg ofbody weight) and two flounders (700 g of body weight) were put in thetank in which N₂ O and CO₂ had been previously dissolved. The pH of seawater was 6.0 and dissolved oxygen was 8.8 to 9.0 ppm.

As the results, it was confirmed that sea breams were put underanesthesia after 2 to 3 minutes and flounders were put under anesthesiaafter 3 to 5 minutes. All fish did not show vigorous swimming motions.

When the aquatic animals put under anesthesia in the above Examples 1 to3 were transferred into ordinary sea water adjusted to DO of at least 7to 8 ppm only by aeration with O₂, all of them completely awakened fromanesthesia within 3 to 5 minutes, although there were individualdifferences.

No animal died in these anesthetization experiments under the proper pHand DO conditions.

EXAMPLE 4

In this Example, anesthetization is carried out by using an apparatus asshown in FIG. 2.

In the apparatus of FIG. 2, a mixed gas of N₂ O and CO₂ are bubbled intoa sea water tank (1) containing aquatic animals from a mixed gas bomb(2) having a control valve and a flowmeter through an aeration pipe (3).O₂ gas is also bubbled from a O₂ bomb (4) through an aeration pipe (5).The tank (1) is equipped with a DO meter (6) and a pH meter (7) tomonitor pH and DO conditions.

Sea water was filled in the tank (1) having the inner volume of about1.5 m³ and three sea breams (about 1.5 kg of body weight) and threeblowfishes (Fugu rubripes, about 500 g of body weight) were put in thetank. The tank was aerated with 30 liter/min. of a mixed gas of CO₂(10%) and N₂ O (90%). O₂ gas was also bubbled into sea water withmonitoring DO meter (6) so as to keep dissolved oxygen at aconcentration of at least 10 ppm.

After 30 minutes, one sea bream rose to the surface lying on its back atpH about 6.2 and it was confirmed that it was put under anesthesia.Thereafter, the fish were put under anesthesia one after another calmlyand, after 40 minutes, all fish were put under anesthesia at pH of about6.0 without vigorous swimming motions.

Then, aeration with the mixed gas of N₂ O and CO₂ was discontinued.While a small amount of O₂ was bubbled so as to keep the concentrationof DO of at least 7 to 8 ppm, the entire water tank was transportedabout 150 km distance over 4 hours by a motor truck.

After transport, the state of each fish was scarcely different from thatjust after putting under anesthesia, although 4.5 hours already elapsed.When the fish were transferred into ordinary sea water aerated with onlyO₂, three blowfishes and one sea bream completely awakened fromanesthesia within several minutes. The reamining two sea breamscompletely awakened from anesthesia after several ten minutes, althoughthey showed "surfacing" like behavior.

EXAMPLE 5

Various kinds of aquatic animals were put in each water tank as shown inFIG. 1 (A: 1.5 m³, B: 650 liter, C: 400 liters, D: 100 liter) andexperiments were carried out according to the same manner as describedin Example 1. The results are shown in Table 3.

As seen from Table 3, the method of the present invention is useful foranesthetizing various aquatic animals.

                                      TABLE 3                                     __________________________________________________________________________                               pH at                                                                              Time                                                                     initial                                                                            required                                                   Size          anesthe-                                                                           for anesthe-                                               Length                                                                            Weight                                                                            Ratio of                                                                            tization                                                                           tization                                      Animals      (cm)                                                                              (kg)                                                                              CO.sub.2 :N.sub.2 O                                                                 stage                                                                              (min.)                                                                              Tank                                    __________________________________________________________________________    Pisces                                                                        sea bream (Chrysophrys                                                                     30  1.0-                                                                              1:8-1:10                                                                            5.8-6.2                                                                            10-20 A, B                                    major)           1.5                                                          flounder (Paralichtys                                                                      30  0.7 1:8-1:10                                                                            5.8-6.2                                                                            10-20 B, C                                    olivaceus)                                                                    yellowtail (Seriola                                                                        50- 3-  1:10-1:30                                                                           6.3-6.8                                                                            15-25 A, B                                    quinqueradiata)                                                                            60  5                                                            filefish (Stephanolepis                                                                    15  0.2 1:10-1:30                                                                           6.3-6.8                                                                             5-10 C                                       cirrhifer)                                                                    mackerel (Scomber                                                                          25  0.5 1:8-1:10                                                                            6.0-6.5                                                                            20-30 D                                       japonicus)                                                                    horse mackerel (Trachurus                                                                  20  0.2 1:8-1:10                                                                            6.0-6.5                                                                            10-20 D                                       japonicus)                                                                    scorpion fish (Sebastiscus                                                                 20  0.3 1:8-1:10                                                                            6.3-6.8                                                                            10-20 D                                       marmoratus)                                                                   sardine (Sardinops                                                                         15  0.1 1:8-1:10                                                                            6.3-6.8                                                                            10-20 D                                        melanosticta)                                                                blowfish (Fugu                                                                             20  0.5 1:8-1:10                                                                            5.8-6.2                                                                            10-20 A                                       rubripes)                                                                     carp (Cyprinus                                                                             20  0.7 1:8-1:10                                                                            4.3-4.5                                                                            30-60 D                                       carpio)                                                                       crucian carp (Carassius                                                                    15  0.5 1:8-1:10                                                                            3.5-4.5                                                                            30-60 D                                       auratus)                                                                      ayu (Plecoglossus                                                                          15  0.2 1:8-1:10                                                                            4.5-5.0                                                                            30-40 D                                       altivelis)                                                                    goldfish      5  --  1:8-1:10                                                                            3.5-4.5                                                                            30-60 D                                       Cephalopoda                                                                   squid (Dorytenthis                                                                         20  0.3 1:10-1:30                                                                           6.3-6.8                                                                            10-20 C                                       kensaki)                                                                      octopus (Octopus                                                                           20  0.3 1:10-1:30                                                                           6.3-6.8                                                                            10-20 C                                       vulgaris)                                                                     Crustacea                                                                     lobster (Panulirus                                                                         20  0.3 1:8-1:10                                                                            5.6-6.0                                                                            20-30 C                                       japonicus)                                                                    prawn (Penaeus                                                                             15  --  1:8-1:10                                                                            5.8-6.2                                                                            20-30 C                                       japonicus)                                                                    swimming crab (Neptunus                                                                    15  --  1:8-1:10                                                                            5.8-6.2                                                                            20-30 C                                       trituberculatus)                                                              river crab (Potamon                                                                         5  --  1:8-1:30                                                                            5.8-6.2                                                                            15-25 D                                       dehaani)                                                                      __________________________________________________________________________

EXAMPLE 6

In this Example, anesthetization is carried out by using an apparatus asshown in FIG. 3.

In the apparatus of FIG. 3, O₂ gas and N₂ O gas are bubbled into a 1stwater tank (1) containing fry from an O₂ bomb (2) and a N₂ O bomb (3),each of which has a control valve and a flowmeter, through aerationpipes (5) and (4), respectively. Likewise, O₂ gas and CO₂ gas arebubbled into a 2nd water tank (6) from the above O₂ bomb (2) and a CO₂bomb (7) through aeration pipes (5) and (8), respectively. The 1st tank(1) is equipped with a DO meter (9) and the 2nd tank (6) is equippedwith a DO meter (9) and a pH meter (10) to monitor pH and DO conditions.

Ten sea bream fry (about 10 cm of body length) were put in the 1st tankfilled with 30 liter of sea water at a temperature of 23° C. at pH 8.0.O₂ gas was intermittently bubbled into the tank so as to keep 5 ppm ofdissolved oxygen by monitoring with the DO meter. Then, N₂ O gas wasbubbled into the tank at the flow rate of 1.5 liter/min until sedativeeffect on the fry was exerted. As the result of aeration with N₂ O,after 15 minutes, initial effect of N₂ O was exerted and relativelyactive swimming motions was observed in comparison with a control groupof fry in an ordinary water tank. After 20 minutes, the fry began torise to the surface and thereby sedative effect of N₂ O was confirmed.That is, the fry put their heads down and their tail fins up and,following the ascending current due to bubbling from the aeration pipes,they rose to the surface. Then, they intended to sink toward the bottomby themselves. They repeated these motions several times. Thereafter,some of them laid on their sides. However, they were not put underanesthesia only by aeration with N₂ O gas.

After bubbling N₂ O gas at the flow rate of 1.5 liter/min. for 35minutes, the fry were transferred to the 2nd tank. In the 2nd tank, CO₂was bubbled at the flow rate of 1.0 liter/min. for about 3 minutes tolower pH of sea water from 8.0 to 5.9. As the result, all the fry werecalmly put under anesthesia without vigorous swimming motions asobserved in the control group which was not treated with N₂ O.

EXAMPLE 7

By using the same apparatus used in Example 6, N₂ O gas was bubbled intothe 1st tank at the flow rate of 1.5 liter/min. for 35 minutes. Then,ten sea bream fry were put therein and allowed to stand for 20 minutes.Thereafter, the fry were transferred into the 2nd tank into which CO₂gas had been bubbled at the flow rate of 1.0 liter/min. for about 3minutes to lower pH to 5.9. As the result, after 1.5 minutes, one fryshowed vigorous swimming motion. However, after 3 minutes, all fry wereput under anesthesia.

COMPARATIVE EXAMPLE

About 350 liter of sea water was filled in the 400 liter tank (1) asshown in FIG. 1 and, according to the same manner as described inExample 1, two yellowtails (about 5 kg of body weight), two sea breams(about 1 kg of body weight) and two flounders (about 700 g of bodyweight) were put in the tank. Only CO₂ was bubbled into the tank at theflow rate of 1.5 liter/min. O₂ was also bubbled into the tank smallporitons by monitoring with DO meter. Behavior of fish were observedaccording to change of pH by comparing with the results in Example 1.The relation of aeration time, pH and DO is shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Time (min.)                                                                              0     2        3   5       7   10                                  pH         6.7   6.1      6.0 5.8     5.6 --                                  DO (ppm)   6.0   6.3      6.8 6.9     6.6 --                                  ______________________________________                                    

From immediatey after commencement of aeration, all fish began to show"surfacing" and vigorous swimming motions. Particularly, yellowtailsjumped up from the tank and were not put under anesthesia, even pHdropped to the prescribed value. At about pH 5.8, all fish were putunder anesthesia. However, yellowtails were damaged such as bleedingfrom the gills and, when they were transferred to ordinary sea water atthe end of the experiment, one of yellowtails did not awaken fromanesthesia and died.

What is claimed is:
 1. A method for anesthetizing aquatic animals whichcomprises anesthetizing the animals in water with nitrous oxide andcarbon dioxide.
 2. A method according to claim 1, wherein nitrous oxideand carbon dioxide are bubbled into water simultaneously.
 3. A methodaccording to claim 1, wherein a mixed gas of nitrous oxide and carbondioxide is bubbled into water.
 4. A method according to claim 1, whereinthe animals are put in water into which nitrous oxide and carbon dioxidehave been previously bubbled.
 5. A method according to claim 1, whereinthe animals are put in water into which nitrous oxide is bubbled andthen they are transferred to another water into which carbon dioxide isbubbled.
 6. A method according to claim 1, wherein the animals are putin water into which nitrous oxide has been previously bubbled and thenthey are transferred to another water into which carbon dioxide has beenpreviously bubbled.
 7. A method according to claim 1, wherein theanimals is those belonging to the class Pisces.
 8. A method according toclaim 1, wherein the animals is those belonging to the class Cephalopodaor Pelecypoda.
 9. A method according to claim 1, wherein the animals isthose belonging to the class Crustacea.
 10. A method according to claim1, wherein the ratio of CO₂ :N₂ O is in the range of 1:1 to 1:100.